1. Field of the Invention
The present invention relates generally to the field of infrared detectors and, more particularly, to particular infrared detecting devices for continuously monitoring and detecting hot air leaks in the bleed air duct of a jet aircraft engine.
2. Description of the Prior Art
Jet aircraft, particularly military fighter planes, fly at high altitudes in which the ambient air temperature is fairly cold, necessitating a defrosting system for the cockpit windshield and an environmental control system in the aircraft which is capable of heating the cockpit environment. The environmental control system, which is typically located adjacent the cockpit area, utilizes hot bleed air taken from the compressor of the jet engine.
A relatively small volume of air at high pressure nad high temperature is fed from the compressor section of the jet engine powering the aircraft. This air, since it is removed at a stage before combustion occurs in the engine, is suitable for use in the environmental control system.
A bleed air duct directs the hot bleed air from the engine, typically located in the rear of the aircraft, to the environmental control system which, as stated earlier, is adjacent the cockpit area near the front of the aircraft. It may therefore be appreciated that the bleed air duct carrying hot, pressurized air extends along a major portion of the fuselage of the aircraft.
Located in the fuselage of the aircraft and necessarily very near to the bleed air duct are complex electronics systems typically including the flight control systems, avionics display systems, communications equipment, and radar and weapons system components, as well as hydraulic controls, fuel lines and fuel tanks. Many of these systems and their components are highly susceptible to damage by exposure to excessive heat. This consideration makes it important to detect promptly any leak which may occur in the bleed air duct in order to minimize the possibility of serious damage or possible loss of the aircraft and pilot.
At present, two types of systems are known which have application to detecting air temperature, which systems may be considered for the purpose of detecting leaks in the bleed air duct system. The first type of system involves placing thermocouples at multiple locations along the length of the bleed air duct. Each of these thermocouples requires a pair of wires to connect it to an amplifier which may be multiplexed between several hundred thermocouples throughout an aircraft. Following the amplifier would be a threshold stage to detect when the thermocouple voltage has exceeded what corresponds to a selected temperature. Due to the number of wires and connections, and due to the low voltage level (and corresponding noise problems) of the thermocouple signal, this method is not generally used except perhaps for testing under controlled conditions.
The second type of system, and the one which has found use in jet aircraft, is the thermal wire system. This system, which is generally regarded as state-of-the art bleed air leak sensor technology, involves the placement of thermal wire in locations where a bleed air leak is most likely to occur. Should the thermal wire be exposed to a direct hot air leak from the bleed air duct, a warning will register.
Unfortunately, the thermal wire warning system will usually only function when a hot air leak is directed onto the thermal wire. When the leak is not directed onto the thermal wire, detection of the leak, if and when it is made, will be slow to develop, which could prevent detection until well after the bleed air leak has occurred, allowing sufficient time for aircraft systems to be damaged or destroyed. The military has reported incidents in which jet aircraft bleed air leaks went totally undetected, and aircraft are known to have sustained considerable damage because of a bleed air leak problem. It may therefore be appreciated that a more reliable bleed air leak detection system is needed.
Thus, it can be seen that there exists a strong need for a detection system capable of quickly and reliably detecting a bleed air leak before equipment located in the congested fuselage can be damaged or destroyed. Such a system must be capable of operating in the congested area, therefore requiring little space and keeping additional weight to a minimum. The bleed air leak sensor system should be capable of identifying a bleed air leak, even in circumstances when the leak is not directed onto the system sensors.
Unlike radiometer systems, the device must be capable of detecting leaks having a very small diameter. It is also desirable that the bleed air leak sensor system be capable of providing a continuous alarm signal as long as the bleed air leak condition continues; in other words, the leak sensor system must continue to function even in cases when the leak is directed onto the sensor itself. Since bays within military aircraft have airflow rates varying from minimal to substantial, depending on the aircraft, the bleed air leak sensor system must be capable of functioning in all types of airflow conditions. Finally, it is desirable that the system provide a warning to the pilot while the plane is still on the ground, thus requiring the system to detect an existing leak even with the engine idling or at least during taxiing or engine run-up prior to take-off.